1. Field of Invention
The present invention relates to methods for preparing micron sized multichromal beads, in particular micron sized bichromal beads, of the type for use in gyricon or electric paper displays (e.g., reimageable paper).
2. Description of Related Art
Reimageable displays, also called gyricon displays, twisting-ball displays, rotary ball displays, particle displays, dipolar particle light valves, reimageable paper, etc., offer a technology for making a form of electric paper. Briefly, a reimageable display is an addressable display made up of a multiplicity of optically anisotropic spheres, each of which can be selectively rotated to present a desired face to an observer. For example, a reimageable display can incorporate spheres each having two distinct hemispheres, one black and the other white (and thus the spheres are bichromal), with each hemisphere having a distinct electrical characteristic (e.g., zeta potential with respect to a dielectric fluid) so that the spheres are electrically as well as optically anisotropic.
In one example of a reimageable display, the bichromal spheres are embedded in a sheet of optically transparent material, such as an elastomer layer, that contains a multiplicity of spheroidal cavities and is permeated by a transparent dielectric fluid, such as a plasticizer. The fluid-filled cavities accommodate the spheres, one sphere per cavity, so as to prevent the spheres from migrating within the sheet. A sphere can be selectively rotated within its respective fluid-filled cavity, for example by application of an electric field, so as to present either the black or the white hemisphere to an observer viewing the surface of the sheet. Thus, by application of an electric field addressable in two dimensions (as by a matrix addressing scheme), the gyricon spheres rotate in response thereto, thereby presenting either a black or white side surface to the viewer. See, for example, U.S. Pat. Nos. 5,262,098, 5,344,594, 5,389,945, 5,717,514, 5,815,306, 5,989,629, 6,097,531, 6,396,621 and 6,441,946, each incorporated herein by reference.
U.S. Pat. No. 6,445,490, incorporated herein by reference, describes another type of bichromal gyricon display sphere. This patent describes a particulate encapsulated gyricon element that includes a gyricon sphere encapsulated within a shell that also contains a dielectric fluid in which the gyricon sphere is able to rotate. Display elements such as electric paper may then be readily derived by coating the encapsulated gyricon elements upon any suitable substrate. This gyricon element has the advantage of not requiring a separate matrix with fluid-filled cavities.
While various reimageable display sheets are known, methods of making the gyricon spheres for such displays could still be improved. Existing methods for forming gyricon spheres include:
U.S. Pat. No. 6,054,071, incorporated herein by reference, describes forming twisting balls (10) for an electric-paper display from a polarizable material and encapsulated in a protective shell (12). The balls (10) are first disposed between first and second electrodes (14, 16) that generate an electric field. The electric field polarizes each ball (10), transforming each into a dipole electret having a first and second charged poles. At least one colorant (20) is then externally applied to each of the balls (10) to obtain multichromal balls such that the charged poles are distinct from each other. The polarized multichromal balls may then be used in fabricating an electric-paper display by encapsulating the balls (10) in an array such that each ball is capable of rotating in response to a selectively applied electric field. See the Abstract. It is also described at column 5, lines 9–47 that the balls may have a core of a dye of one color and a colorant of a different color, with the colorant moving to one side of the ball under the influence of an electric field.
U.S. Pat. No. 5,344,594, incorporated herein by reference, describes a method of forming hemispheric bichromal balls, including the steps of bringing together two streams of differently colored hardenable liquids for forming a single side-by-side bichromal stream, expelling the single side-by-side bichromal stream into a fluid as one or more free jets whose forward ends becomes unstable and break up into droplets which form into spherical balls, each of the balls comprising hemispheres of differently colored hardenable liquid, and finally hardening the balls.
U.S. Pat. No. 5,262,098, incorporated herein by reference, describes a method similar to that described in U.S. Pat. No. 5,344,594. This reference further describes, in FIGS. 8 and 9, that an electric field may be applied during formation of the balls in order to prevent premature collision of the balls. This field is thus applied while the balls are moving, and therefore cannot be used to arrange colored particles within the balls.
The aforementioned methods of making gyricon spheres have several drawbacks. For example, it is difficult to consistently achieve smaller sizes of gyricon spheres by these methods. Known methods of making gyricon spheres typically form gyricon spheres of about 100 microns in diameter at a minimum. However, high resolution applications require that gyricon spheres have an average diameter on the order of 50 microns or less.
Moreover, the known methods of making gyricon spheres often result in a high number of unusable spheres, i.e., spheres in which the colors are not properly distributed, in which no core at all is present, etc. The methods thus have large inefficiencies.
Thus, it is desired to develop a novel method of making gyricon spheres that improves and/or addresses some or all of the aforementioned drawbacks of the known methods.